TNRTB Classic: The Rise in Free Oxygen

Scientists realize that the existence of certain elements depends on a delicate balance in how the physical laws manifest themselves. Three of those elements (oxygen, carbon, and hydrogen) also figure prominently in how life operates and develops. Earth contains an abundance of oxygen—in water (H2O) for example—yet no free oxygen existed for a couple billion years after Earth formed. Starting around 2.5 billion years ago, free oxygen gained a foothold in the atmosphere and also showed dramatic increases in the intervening time. Often, the life on Earth showed a similarly dramatic change in response to the oxygen changes. For some examples, see these previous articles:

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